auxdisplay: linedisp: display static message when length <= display size

auxdisplay: linedisp: display static message when length <= display size

Currently, when a message shorter than the display size is written, the
content wraps around (e.g., "123" on a 4-digit display shows "1231")
without scrolling, which is confusing and unintuitive.

Change behavior to display short messages statically with space padding
(e.g. "123 ") while only scrolling messages longer than the display width.
This provides more natural behavior that aligns with user expectations
and current linedisp_display() kernel-doc.

The scroll logic is also consolidated into a helper function for clarity.

No API changes are introduced.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>